This invention relates to the removal of solid particles from a liquid. In one of its aspects the invention relates to apparatus for controlling the operation of an electrostatic filter. In a further aspect the invention relates to a process for the removal of catalyst particles from a liquid hydrocarbon fraction in a catalytic cracking process.
In the refining of crude oil it is sometimes necessary to remove finely-divided solid particles present in the oil or introduced into a hydrocarbon fraction of the oil during processing. An example of a process in which removal of solid particles from a liquid hydrocarbon is necessary is catalytic cracking, the treatment of petroleum to convert hydrocarbons having a relatively high molecular weight into hydrocarbons of lower boiling range. In this process, the product from the cracking reactor is fractionated to recover light hydrocarbon gasoline and various grades of cycle oil. In the fractionation process, a slurry oil which can be used as a carbon black feedstock is produced. This slurry oil contains catalyst particles which must be removed to provide a suitable carbon black feedstock as well as to prevent waste of the catalyst, which can be returned to the cracking reactor and reused.
The presently preferred method for removing such finely-divided particles from a hydrocarbon fraction is to pass the hydrocarbon through electrostatic filters such as those described in U.S. Pat. No. 4,059,498. In these filters, the suspended solid particles are collected on a bed of glass beads in a high voltage direct current field. After a period of operation, the efficiency of particle removal begins to decline because of the accumulation of solid particles in the filter. The collected particles are removed by taking the filter off stream and backflushing the filter with a liquid. It is often desirable to use the unfiltered feed hydrocarbon as the backflushing medium, as the use of filtered product reduces the overall capacity of the filtering operation. It has been found that resumption of the on-stream operation of the filter following backflushing can be accompanied by contamination of the filtered product stream by solid particles.
It is therefore an object of this invention to provide an improved process and apparatus for operation of an electrostatic filter.
It is a further object of the invention to enable the use of unfiltered fluid as the filter backflush medium without the usual accompanying contamination of the filtered product stream.